1. Field of the Invention
The present invention is directed to a method for suppressing stimulations in a living examination subject as a result of switched gradient fields during magnetic resonance tomography scans of areas of the examination subject that lie within the imaging volume of the magnetic resonance tomography device.
2. Description of the Prior Art
Magnetic resonance tomography is a known technique for acquiring data from the interior of the body of a living patient. Rapidly switched gradient fields of high amplitude are superimposed on a static basic magnetic field of high amplitude to obtain such data.
In a magnetic resonance image scan (data acquisition procedure) stimulations can be initiated in living examination subjects due to the switching of the gradient fields. The gradient fields affecting the examination subject are characterized by a magnetic flux density, which changes in terms of time. The time-changing magnetic field generates eddy currents or induction currents in the examination subject. Their nature is mainly dependent on the shape and size of microscopic structures in the body. Due to their mutual electromagnetic interaction, these currents influence physiological currents, such as potentials at cells. All cells have an equilibrium rest potential. At the equilibrium rest potential, all membrane currents of a cell are in equilibrium. When the membrane potential is depolarized by means of an additional membrane current, which, for example, reaches the cell from an outside influence, this is associated with a change of the potential, known as an action potential. The initiation potential for an action potential is named the threshold. The equilibrium of the membrane currents changes at the threshold. Additional membrane currents, which depolarize the membrane, occur for a short period of time. An action is associated with an action potential. For example, every twitching of a muscle fiber is accompanied with an action potential in the muscle fiber, and each reaction of a sense cell to a sense stimulus is forwarded by means of action potentials. For a detailed description of neurophysiological processes, see the book of R. F Schmidt (publisher) xe2x80x9cNeuround Sinnesphysiologiexe2x80x9d, Springer 2nd corrected edition 1995, chapters 2 and 3.
The intensity of the electrical currents generated by the switched gradient fields, among other things, depends on the cross-sectional surface which is permeated by the gradient field and depends on the time change of the gradient field. The aforementioned currents traverse regions of the examination object with a comparatively high electrical conductivity as well as regions of low electrical conductivity. Depending on whether areas with high or low electrical conductivity are concerned, the above cited current effects a correspondingly large or small electrical voltage in these regions. When the voltage exceeds a specific threshold, this leads to the initiation of action potentials and the examination subject is stimulated.
In order to avoid such stimulations in the examined body with respect to rapidly switched gradient fields, it is known from German OS 42 25 592 to cover stimulation-sensitive areas outside of the imaging volume with a closed conductor loop. This leads to a reduction of the currents induced in the covered area. This method is based on the knowledge that the highest current values are induced outside of the imaging volume with respect to switched gradient fields, so that the danger of stimulations is greatest there. As a result of attaching conductor loops outside of the imaging volume, however, the linearity of the gradients, which is important for the image quality, is somewhat degraded and the position of the conductor loops normally must be adapted as well with respect to only changes of the imaging area.
Further, a series of methods is known that make it possible to predict or calculate stimulations in an examination subject undergoing magnetic resonance tomography. A detailed description can be found in German OS 199 13 547, for example. For example, stimulations can be avoided in by suitably controlling the curves of the gradient fields, however, this may mean that the best efficiency of the gradient system is not fully utilized.
An object of the present invention is to provide a method and a device for implementing the method, for suppressing stimulations of an examination subject, so that above cited disadvantages are avoided or at least reduced.
The object is inventively achieved in a method and device wherein at least two electrodes, which are connected to one another by at least one electrically conductive connection, are attached to the skin of the examination subject for bridging at least one area of the examination subject, which exhibits a lower electrical conductivity than the areas surrounding it.
A parallel current path is thereby created for the currents in areas of low electrical conductivity of a patient, which currents are generated by the gradient fields, so that the currents and therefore also the arising electrical voltages, are reduced in these areas, so that stimulations are prevented.
In an embodiment, the area of low electrical conductivity is situated outside of the imaging volume. The area in which the danger of stimulation is greatest is protected as a result.
In another embodiment, the transition resistance between the electrode and the skin is made close to zero, for example by applying an electrically conductive fluid or gel between the electrode and the skin. Injuries of the patient at the transition location of the current from the skin to the electrode as a result of an undesired, large voltage drop at the transition location are thereby prevented.
In a further embodiment, the electrically conductive connection between the electrodes exhibits an electrical conductivity on the order of magnitude of electrically highly conductive areas of the examination subject, for example in the range of 0.2 S/m through 4 S/m. This prevents a parallel current path of such low impedance from being created opposite the cited area. Such a current path would initiate current densities at the current transition locations between skin and electrodes, and these densities would lead to injuries of the patient.
In another embodiment, the device for implementing the method is flexibly changeable in shape, so that an adaptation to differently shaped areas of the examination subject is possible. For this purpose, the device is fashioned as a pillow or cushion in a preferred embodiment. The pillow contains the electrode areas, which can be accessed at the pillow surface, and contains the electrically conductive connection between the electrode areas. For this purpose, the pillow contains, for example, an electrically conductive pillow case and an electrically conductive pillow filling, for example a saline solution, or it contains a gel-like mass.